Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Referring to FIG. 1, typically a turbofan gas turbine engine 1010 will comprise an axial fan 1012 driven by an engine core 1015. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The turbofan gas turbine engine 1010 is generally provided with outlet guide vanes (OGVs) 1032 downstream of the fan 1012 for straightening flow from the fan. Typically one or two bifurcations 1038 are provided downstream of the OGVs between the inner and outer walls of the bypass duct. The bifurcations include aerodynamically shaped fairings around the pylon structure for the purpose of encasing and directing electrical connections. Usually one bifurcation is positioned between the engine core 1015 and a pylon 1008 that mounts the engine to the wing of an aircraft. If two bifurcations are provided, the second bifurcation is generally positioned diametrically opposite the bifurcation shown in FIG. 1.
However, the bifurcation provides a substantial and undesirable blockage to the flow downstream of the OGVs. This means that part of the flow from the OGVs stagnates at the bifurcation leading edge. The stagnated flow reaches a high static pressure equal to the incoming total pressure, also called stagnation pressure. The remaining flow from the OGVs flows around the bifurcation which results in an acceleration, hence a reduction in pressure. The resulting high leading edge stagnation pressure and the low pressure around the area of high pressure bring a significant peak-to-peak variation in the static pressure field seen from the OGV trailing-edge.
This problem is illustrated in FIGS. 2A, 2B and 2C. Referring to FIG. 2A, the static pressure field is indicated at 1054 and the resulting force on the fan is indicated by arrow F. Referring to FIG. 2B, the region between the bifurcation 1038 and the OGVs 1032 (both shown in FIG. 2A) is illustrated. As can be seen in FIG. 2B, an area of high static pressure 1052 is formed in a region corresponding to the position of the two bifurcations, and an area of low pressure 1050 is formed in a region corresponding to a position between (i.e. away from) the two bifurcations. Referring to FIG. 2C, the circumferential static pressure PS variation is plotted against the circumferential position represented as an angle (in degrees) from a 0° position that is substantially aligned with the upper bifurcation centre line. As can be seen the static pressure varies circumferentially between the a maximum at a position substantially aligned with the centreline of the bifurcation and a minimum at a position corresponding to 90° from the centreline of the bifurcation.
One method of addressing this problem is to alter the stagger angle and/or camber of the OGVs to reduce the peak-to-peak variation in the static pressure field as seen by the rotor upstream of the OGV row. However, with this method there is still a level of residual pressure variation upstream of the OGV row. This pressure variation exerts a significant forcing on the fan blades. Furthermore, altering the stagger angle and/or camber of the OGVs can reduce aerodynamic efficiency by introducing increased pressure losses.
Accordingly, there is a need for a solution that reaches a balance between minimising the forcing on the fan blades and maximising the aerodynamic efficiency of the OGVs.